Project Summary With the rapid expansion of the use of immune checkpoint inhibitors (ICI) across a wide variety of histologies, it remains clear that only small minorities of patients respond. There is therefore a large need for improved predictors of response to immunotherapy. In particular, in cancers associated with germline defects in homologous recombination, including breast, ovarian, and prostate cancer, response rates to immunotherapy have been very low in large phase 3 trials, in some cases leading to a decision by pharmaceutical companies to no longer pursue FDA approval for these indications. DNA-repair deficiency could serve as a useful and clinically relevant biomarker to select appropriate patients for immunotherapy. This is reminiscent of the recent FDA approval of pembrolizumab for patients with mismatch repair deficient tumors regardless of histology. A similar concept may extend to other defects in DNA-damage response and repair (DDR) pathways, the most common being homologous recombination (HR). Defects in homologous recombination repair have been studied extensively due to the hereditary predisposition for breast, ovarian prostate, and pancreatic cancer with mutations in important HR genes BRCA1 and BRCA2. More recently, phenotypic determination of homologous recombination deficiency (HRD) using genomic correlates have estimated up to 30% of breast and ovarian cancers may be HRD as well as a small portion of many other histologies. In ongoing work we have implicated mutations in the homologous recombination pathway with improved survival after ICI in a large retrospective cohort. However, divergent responses were noted depending on the HR gene mutated. This proposal aims to further explore the hypothesis that HRD and other DDR defects result in increased immunogenicity and as a result improved response to immunotherapy and elucidate the mechanisms involved in resulting immune recognition and response to ICI. Aim 1 will focus on gaining an understanding of which genes are associated with response to ICI and explain the divergent responses observed clinically. A thorough analysis of the immune microenvironment will result in important insights regarding which patients my benefit and why. Aim 2 will attempt to elucidate what components of the immune system are directly influenced by HRD and DDR deficiencies focusing on innate immunity pathways. Aim 3 will study the effects of other DDR pathway deficiencies or activation using drugs or radiation and their affects on response to ICI, exploring potential synergy.